The present invention relates to catalysts, and more specifically, to transition metal catalysts useful for hydrogenation reactions.
Transition metal catalysts are well-known in the prior art. Some of these catalysts have been synthesized and used in heterogeneous form. For example, the transition metal complex can be attached to a polymeric support by a linking agent. Heterogeneous catalysts have the advantage of facilitating or eliminating the catalyst recovery operations that are necessary when homogeneous catalysts are used, while retaining the advantages of high activity, selectivity, and reproduceability under mild temperatures and pressures.
The catalysts of particular interest here have transition metal complexes bonded to a polymeric support by a phosphorus containing group. Apparently all catalysts of this type used in the past have had a dative bond between the phosphorus atom and transition metal. This can be illustrated by a typical synthesis of such a compound.
The synthesis below begins with a chloromethylated styrene-divinylbenzene copolymer (1) and lithium diphenylphosphine (2). ##STR1##
The resulting polymer supported phosphine (3) contains PPh.sub.2 units which can then be attached to various transition metals, such as rhodium. ##STR2##
A triphenylphosphine group is lost from ClRh(PPh.sub.3).sub.3 and the resulting polymeric material (4) is a catalyst with many applications.
The catalyst described above has a costly disadvantage: the dative two electron bond between the phosphorus and the rhodium is fairly weak. When such a catalyst is used in the harsh environment of commercial operations, a significant amount of rhodium will be lost from the polymeric support. Rhodium and several other transition metals commonly used as catalysts are extremely expensive, so their loss during operations can make otherwise satisfactory catalytic processes economically unfeasible.
Furthermore, prior art catalysts generally require high temperatures and pressures for the hydrogenation of aromatic hydrocarbons. Fifty atmospheres and 150.degree. C. are not uncommon requirements, and some catalysts and reactants require even harsher conditions.
A catalyst less vulnerable to transition metal loss will provide a significant advantage in commercial operations. The same is true of one that could hydrogenate unsaturated hydrocarbons under milder conditions.